This invention relates in general to electrophotographic imaging members and, more specifically, to layered photoreceptor structures with overcoatings containing hydrogen bonded materials and processes for making and using the photoreceptors.
Electrophotographic imaging members, i.e. photoreceptors, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the dark so that electric charges are retained on its surface. Upon exposure to light, the charge is dissipated.
A latent image is formed on the photoreceptor by first uniformly depositing an electric charge over the surface of the photoconductive layer by one of any suitable means well known in the art. The photoconductive layer functions as a charge storage capacitor with charge on its free surface and an equal charge of opposite polarity (the counter charge) on the conductive substrate. A light image is then projected onto the photoconductive layer. On those portions of the photoconductive layer that are exposed to light, the electric charge is conducted through the layer reducing the surface charge. The portions of the surface of the photoconductor not exposed to light retain their surface charge. The quantity of electric charge at any particular area of the photoconductive surface is inversely related to the illumination incident thereon, thus forming an electrostatic latent image.
The photodischarge of the photoconductive layer requires that the layer photogenerate conductive charge and transport this charge through the layer thereby neutralizing the charge on the surface. Two types of photoreceptor structures have been employed: multilayer structures wherein separate layers perform the functions of charge generation and charge transport, respectively, and single layer photoconductors which perform both functions. These layers are formed on an electrically conductive substrate and may include an optional charge blocking and an adhesive layer between the conductive layer and the photoconducting layer or layers. Additionally, the substrate may comprise a non-conducting mechanical support with a conductive surface. Other layers for providing special functions such as incoherent reflection of laser light, dot patterns for pictorial imaging or subbing layers to provide chemical sealing and/or a smooth coating surface may be optionally be employed.
One common type of photoreceptor is a multilayered device that comprises a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. The charge transport layer can contain an active aromatic diamine molecule, which enables charge transport, dissolved or molecularly dispersed in a film forming binder. This type of charge transport layer is described, for example in U.S. Pat. No. 4,265,990. The disclosures of this patent is incorporated herein in its entirety. Other charge transport molecules disclosed in the prior art include a variety of electron donor, aromatic amines, oxadiazoles, oxazoles, hydrazones and stilbenes for hole transport and electron acceptor molecules for electron transport. Another type of charge transport layer has been developed which utilizes a charge transporting polymer wherein the charge transporting moiety is incorporated in the polymer as a group pendant from the backbone of the polymer backbone or as a moiety in the backbone of the polymer. These types of charge transport polymers include materials such as poly(N-vinylcarbazole), polysilylenes, and others including those described, for example, in U.S. Pat. Nos. 4,618,551, 4,806,443, 4,935,487, and 4,956,440. The disclosures of these patents are incorporated herein in their entirety. Other charge transporting materials include polymeric arylamine compounds and related polymers described in U.S. Pat. Nos. 4,801,517, 4,806,444, 4,818,650, 4,806,443, 5,030,532, copending application Ser. No. 07/797,753, now U.S. Pat. No. 5,283,143, entitled xe2x80x9cELECTROPHOTOGRAPHIC IMAGING MEMBER CONTAINING ARYLAMINE TERPOLYMERS WITH CF3 SUBSTITUTED MOIETIESxe2x80x9d, to Yanus et al, filed Nov. 25, 1991 and copending application Ser. No. 08/148,818, now U.S. Pat. No. 5,356,943, to Yanus et al, entitled xe2x80x9cELECTROPHOTOGRAPHIC IMAGING MEMBERS CONTAINING POLYARYLAMINE POLYESTERSxe2x80x9d, mailed by Express Mail on Nov. 8, 1993, the disclosures of which are also incorporated herein by reference in their entirety.
One of the design criteria for the selection of the photosensitive pigment for a charge generator layer and the charge transporting molecule or polymer for a transport layer is that, when light photons photogenerate holes in the pigment, the holes be efficiently injected into the charge transporting moiety in the transport layer. More specifically, the injection efficiency from the pigment to the transport layer should be high. A second design criterion is that the injected holes be transported across the charge transport layer in a short time; shorter than the time duration between the exposure and development stations in an imaging device. The transit time across the transport layer is determined by the charge carrier mobility in the transport layer. The charge carrier mobility is the velocity per unit field and has dimensions of cm2/volt sec. The charge carrier mobility is a function of the structure of the charge transporting moiety, the concentration of the charge transporting moiety in the transport layer and the electrically xe2x80x9cinactivexe2x80x9d binder polymer in which the charge transport molecule is dispersed (if the transport layer consists of charge transporting molecules dispersed in a binder). It is believed that the injection efficiency can be maximized by choosing a transporting moiety whose ionization potential is lower than that of the pigment (assuming the charge transporting carriers are holes). However, low ionization potential molecules may have other deficiencies, one of which is their instability in an atmosphere of corona effluents. A copy quality defect resulting from the chemical interaction of the surface of the transport layer with corona effluents is referred to as xe2x80x9cparking deletionxe2x80x9d and is described in detail below.
Photoreceptors are cycled many thousands of times in automatic copiers, duplicators and printers. This cycling causes degradation of the imaging properties of photoreceptors, particularly multilayered organic photoconductors which utilize organic film forming polymers and small molecule low ionization donor material in the charge transport layers. Such wear is accelerated when the photoreceptor is utilized in systems employing abrasive development systems such as single component development systems. Wear is an even greater problem where a drum is utilized which has such a small diameter that it must rotate many, many times merely to form images for each conventional size 8.5 inch by 11 inch document. Wear of the photoreceptor can be compensated by increasing the thickness of the charge transport layer. However, large increases in thickness of the charge transport layer can render the photoreceptor inoperable at high imaging process speeds because of the inadequate (very long) transit times of common charge transport layer materials. Also, large decreases in thickness due to wear can cause dramatic changes in electrical characteristics in only a few thousand cycles that cannot be readily compensated by even sophisticated computerized control apparatus.
When the electrophotographic imaging member is utilized in liquid ink development systems, leaching of small molecules from the charge transport layer into the liquid development can occur. Loss of the small molecule material due to leaching causes undesirable deterioration in electrical properties of the photoreceptor. Also, undesirable crystallization of the small molecule in the charge transport layer can adversely affect the electrical imaging characteristics of the photoreceptor.
Reprographic machines utilizing multilayered organic photoconductors also employ corotrons or scorotrons to charge the photoconductors prior to imagewise exposure. During the operating lifetime of these photoconductors they are subjected to corona effluents which include ozone, various oxides of nitrogen, etc. It is believed that some of these oxides of nitrogen are converted to nitric acid in the presence of water molecules present in the ambient operating atmosphere. The top surface of the photoconductor is exposed to the nitric acid during operation of the machine and charge transporting moiety at the very top surface of the transport layer are converted to what is believed to be the nitrated species of the molecules and these could form an electrically conductive film. However, during operation of the machine, the cleaning subsystem continuously removes (by wear) a region of the top surface thereby preventing accumulation of the conductive species. Unfortunately, such is not the case when the machine is not operating (i.e. in idle mode) between two large copy runs. During the idle mode between long copy runs, a specific segment of the photoreceptor comes to rest (is parked) beneath a corotron that had been in operation during the long copy run. Although the high voltage to the corotron is turned off during the time period when the photoreceptor is parked, some effluents (e.g. nitric acid, etc.) continue to be emitted from the corotron shield, corotron housing, etc. This effluent emission is concentrated in the region of the stationary photoreceptor parked directly underneath the corotron. The effluents render that surface region electrically conductive. When machine operation is resumed for the next copy run, image spreading and loss of resolution occurs in the region of the photoconductor where surface conductivity has increased. Deletion may also be observed in the loss of fine lines and details in the final print. Thus, the corona induced changes primarily occur at the surface region of the charge transport layer. These changes are manifested in the form of increased conductivity which results in loss of resolution of the final toner images. Loss of resolution along the entire imaging surface can occur due to an increase in surface conductance caused by corona species interaction. In the case of excessive increases in conductivity, there can be regions of extreme deletions in the images. This problem is particularly severe in devices employing arylamine charge transport molecules such as N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-(1,1xe2x80x2-biphenyl)-4,4xe2x80x2-diamine and charge transport polymers incorporating diamine transporting moiety.
Although, xe2x80x9cparking deletionxe2x80x9d is described above, in some cases deletion might occur in all portions of the photoconductor. This will depend on the number and type of corotrons employed, the design of the photoconductor cavity and air-flow patterns around the photoconductor.
Thus, although the charge transport moiety meets most other electrophotographic criteria such as being devoid of traps, having high injection efficiency from many pigments, ease in synthesizing, and inexpensive, it encounters serious parking and other deletion problems.
U.S. Pat. No. 4,297,425 to Pai et al., issued Oct. 27, 1981xe2x80x94A layered photosensitive member is disclosed comprising a generator layer and a transport layer containing a combination of diamine and triphenyl methane molecules dispersed in a polymeric binder.
U.S. Pat. No. 4,050,935 to Limburg et al., issued Sep. 27, 1977xe2x80x94A layered photosensitive member is disclosed comprising a generator layer of trigonal selenium and a transport layer of bis(4-diethylamino-2-methylphenyl)phenylmethane molecularly dispersed in a polymeric binder.
U.S. Pat. No. 4,457,994 to Pai et al. et al, issued Jul. 3 1984xe2x80x94A layered photosensitive member is disclosed comprising a generator layer and a transport layer containing a diamine type molecule dispersed in a polymeric binder and an overcoat containing triphenyl methane molecules dispersed in a polymeric binder.
U.S. Pat. No. 4,281,054 to Horgan et al., issued Jul. 28, 1981xe2x80x94An imaging member is disclosed comprising a substrate, an injecting contact, or hole injecting electrode overlying the substrate, a charge transport layer comprising an electrically inactive resin containing a dispersed electrically active material, a layer of charge generator material and a layer of insulating organic resin overlying the charge generating material. The charge transport layer can contain triphenylmethane.
U.S. Pat. No. 4,871,634 to Limburg et al., issued Oct. 3, 1989xe2x80x94An electrostatographic imaging member is disclosed which contains at least one electrophotoconductive layer, the imaging member comprising a photogenerating material and a hydroxy arylamine compound represented by a certain formula. The hydroxy arylamine compound can be used in an overcoating with the hydroxy arylamine compound bonded to a resin capable of hydrogen bonding such as a polyamide possessing alcohol solubility.
U.S. Pat. No. 4,515,882 to Mammino et al, issued May 7, 1985xe2x80x94An electrophotographic imaging system is disclosed which utilizes a member comprising at least one photoconductive layer and an overcoating layer comprising a film forming continuous phase comprising charge transport molecules and finely divided charge injection enabling particles dispersed in the continuous phase, the insulating overcoating layer being substantially transparent to activating radiation to which the photoconductive layer is sensitive and substantially electrically insulating at low electrical fields.
U.S. Pat. No. 4,599,286 to Limburg et al., issued Jul. 8, 1982xe2x80x94An electrophotographic imaging member is disclosed comprising a charge generation layer and a charge transport layer, the transport layer comprising an aromatic amine charge transport molecule in a continuous polymeric binder phase and a chemical stabilizer selected from the group consisting of certain nitrone, isobenzofuran, hydroxyaromatic compounds and mixtures thereof. An electrophotographic imaging process using this member is also described.
In copending application entitled xe2x80x9cLAYERED PHOTORECEPTOR STRUCTURES WITH OVERCOATINGS CONTAINING A TRIPHENYL METHANExe2x80x9d Ser. No. 07/997,412, filed on Dec. 28, 1992, refiled as continuation application Ser. No. 08/261,651, now U.S. Pat. No. 5,391,447, an electrophotographic imaging member is disclosed comprising a substrate, a charge generating layer, a charge transport layer comprising charge transporting molecules dispersed in a first polymer binder, and an overcoat layer comprising a triphenyl methane molecule dispersed in a second polymer binder, the second polymer binder being soluble in a solvent in which the first polymer binder is insoluble. The overcoat layer may contain an optional charge transport molecule. The device may also include any suitable optional charge blocking, adhesive and other sub layers. This electrophotographic imaging member is fabricated by forming on a charge generating layer a first coating comprising charge transporting molecules dispersed in a solution of a first polymer binder dissolved in a first solvent, drying the coating to remove the solvent to form a substantially dry charge transport layer, forming on the charge transport layer a second coating comprising triphenyl methane molecules and charge transporting molecules dispersed in a solution of a second polymer binder dissolved in a second solvent, the first polymer binder being insoluble in the second solvent, and drying the second coating to remove the second solvent to form a substantially dry overcoat layer. This electrophotographic imaging member may be utilized in an electrophotographic imaging process. The entire disclosure of this copending application is incorporated herein by reference.
In copending application entitled xe2x80x9cLAYERED PHOTORECEPTOR WITH OVERCOATINGS CONTAINING HYDROGEN BONDED MATERIALSxe2x80x9d, Ser. No. 08,172,520 filed concurently herewith on Dec. 21, 1993, now U.S. Pat. No. 5,368,967 an electrophotographic imaging member comprising a substrate, a charge generating layer, a charge transport layer, and an overcoat layer comprising a small molecule hole transporting arylamine having at least two hydroxy functional groups, a hydroxy or multihydroxy triphenyl methane and a polyamide film forming binder capable of forming hydrogen bonds with the hydroxy functional groups the hydroxy arylamine and hydroxy or multihydroxy triphenyl methane. This overcoat layer may be fabricated using an alcohol solvent. This electrophotographic imaging member may be utilized in an electrophotographic imaging process. The entire disclosure of this copending application is incorporated herein by reference.
In copending application entitled xe2x80x9cPHOTORECEPTOR WITH LOW SURFACE ENERGY OVERCOATxe2x80x9d, Ser. No. 08/170,480, filed concurently herewith on Dec. 21, 1993, now U.S. Pat. No. 5,438,099, an electrophotographic imaging member is disclosed comprising a substrate, a charge generating layer, a charge transport layer, and an overcoat layer comprising a small molecule hole transporting arylamine having at least two hydroxy functional groups, a hydroxy terminated dimethyl siloxane and a polyamide film forming binder capable of forming hydrogen bonds with the hydroxy functional groups on the hydroxy arylamine and hydroxy diorgano siloxane. This overcoat layer may be fabricated using an alcohol solvent. This electrophotographic imaging member may be utilized in an electrophotographic imaging process. The entire disclosure of this copending application is incorporated herein by reference.
Although acceptable images may be obtained when chemical triphenyl methanes are incorporated within the bulk of the charge transport layers, the photoreceptor can exhibit at least two deficiencies when subjected to extensive cycling. One is that the presence of the triphenyl methane in the bulk of the charge transport layer results in trapping of photoinjected holes from the generator layer into the transport layer giving rise to higher residual potentials. This can cause a condition known as cycle-up in which the residual potential continues to increase with multi-cycle operation. This can give rise to increased densities in the background areas of the final images. A second undesirable effect due to the addition of the triphenyl methane in the bulk of the transport layer is that some of these molecules migrate into the generator layer during the process of the fabrication of the transport layer. The presence of these molecules on the surface of the pigment in the generator layer could result in cyclic instabilities, particularly in long image cycling runs. These two deficiencies limits the concentration of the triphenyl methanes that can be added in the transport layer.
Where photoreceptors are overcoated with films containing triphenyl methanes, intermixing of the overcoat and the transport layers occur which can render the overcoat very ineffective. This intermixing leads to the incorporation of hydroxy triphenyl methanes in the bulk of the transport layer causing cycle-up. Also, the intermixing causes a reduction of the concentration of triphenyl methanes on the outer surface of the photoreceptor. The concentration of triphenyl methanes in the outer surface region of the photoreceptor prevents the aforementioned deletion.
Thus, there is a continuing need for photoreceptors having improved resistance to corona effluent induced deletions without increased densities in the background areas of the final images, migration of additives into the generator layer during fabrication of the transport layer, and cyclic instabilities.
It is, therefore, an object of the present invention to provide an improved electrophotographic imaging member which overcomes the above-noted deficiencies.
It is another object of the present invention to provide an improved electrophotographic imaging member exhibiting greater resistance to abrasion during image cycling.
It is yet another object of the present invention to provide an improved electrophotographic imaging member that resists leaching of components from the charge transport layer during liquid development.
It is yet another object of the present invention to provide an improved electrophotographic imaging member that resists cracking of the charge transport layer during liquid development.
It is still another object of the present invention to provide an improved electrophotographic imaging member which reduces crystallization of small molecules in the charge transport layer.
It is another object of the present invention to provide an improved electrophotographic imaging member which is stable against copy defects such as print deletion.
It is yet another object of the present invention to provide an improved electrophotographic imaging member having greater stability against corona induced chemical changes.
It is another object of the present invention to provide an improved electrophotographic imaging member which avoids residual charge build up.
It is still another object of the present invention to provide an improved electrophotographic imaging member which is mechanically stronger.
It is yet another object of the present invention to provide an improved electrophotographic imaging member having an overcoating free of phase separation of component materials.
The foregoing objects and others are accomplished in accordance with this invention by providing an electrophotographic imaging member comprising a substrate, a charge generating layer, a charge transport layer, and an overcoat layer comprising a small molecule hole transporting triphenyl methane having at least one hydroxy functional group, and a polyamide film forming binder capable of forming hydrogen bonds with the hydroxy functional groups of the hydroxy triphenyl methane. This overcoat layer may be fabricated using an alcohol solvent. This electrophotographic imaging member may be utilized in an electrophotographic imaging process.